1. Field of the Invention
The present invention relates to a lumped-element elliptic lowpass filter, and more particularly, to a lumped-element elliptic lowpass filter realized in a multi-layered substrate.
2. Description of the Prior Art
Lowpass filters are widely used as building blocks in circuit designs. They are usually used to filter out unwanted spurious responses and harmonics of higher frequencies. Performance of lowpass filters is characterized by insertion losses in the passband and rejections in the stopband.
Please refer to FIG. 1. FIG. 1 is a prototype circuit of a conventional nth-order lowpass filter 10, where n is an odd number. P1 and P2 are two ports of the lowpass filter 10. The lowpass filter 10 contains inductors L1, L3, L5, . . . , Ln and capacitors C2, C4, . . . , Cn-1. For cases in which the rejection in the stopband of the lowpass filter 10 shown in FIG. 1 is not enough to meet system requirements, an elliptic lowpass filter can be employed.
Please refer to FIG. 2. FIG. 2 is a prototype circuit of a conventional nth-order elliptic lowpass filter 20, where n is an odd number. Compared with the lowpass filter 10 in FIG. 1, in the elliptic lowpass filter 20, additional inductors L2j are added in series with shunt capacitors C2j, where j is a positive integer. The resulting series LC circuits will create notches in the insertion loss response within the stopband, and a better stopband rejection can be achieved.
Please refer to FIG. 3. FIG. 3 is a diagram of frequency responses for a conventional third-order lowpass filter and a conventional third-order elliptic lowpass filter. A solid line 21 represents the insertion loss of the conventional third-order lowpass filter, and a dotted line 22 represents the insertion loss of the conventional third-order elliptic lowpass filter. As shown in FIG. 3, there is a notch in the stopband of the elliptic filter, such that a better stopband rejection is achieved. In addition, the elliptic filter also exhibits sharper roll-off rate at the passband edge, which is usually considered a good characteristic for a lowpass filter.
Although elliptic lowpass filters exhibit much better performance compared to conventional lowpass filters, they require additional inductors. A larger number of circuit elements increases the circuit area needed. This is especially undesirable for modern wireless communication devices that are subject to stringent requirements on size and cost. Therefore, the task of designing a lowpass filter with fewer elements and a good rejection in the stopband has become an important topic in the advanced development of RF circuits.